A wide variety of devices are currently under development utilizing the wider band gap materials such as gallium arsenide or gallium phosphide which provide high intrinsic electron mobility at low fields. Electron mobility of gallium arsenide is between five and six times that of silicon and offers the prospect of greatly increased performance in speed, power or both.
The low field electron mobility of GaAs is one of its greatest attributes and offers high frequency operation in devices such as the field effect transistor (FET). The technology of manufacturing high performance GaAs (FET's is maturing at a rapid rate and the devices are experiencing a greatly expanding role in oscillator, mixer, logic element, power amplification and low noise/high gain applications especially in satellite communications, military weapon systems, IC test equipment and mainframe computers. However, the full potential has yet to be realized by substrate suppliers and device manufacturers Present crystal preparation methods do not provide reliable and reproducible high performance devices.
The wider band gap materials, such as gallium arsenide, are all compounds which are subject to complex binary phase equilibria. In every case, one component of the compound, i.e., arsenic, is volatile. Escape of arsenic from the growing crystal causes crystal dislocation defects and vacancies which affect the parameters of the bulk crystal and also affect the structure and performance of epitaxial layers grown on the surface of the crystal.
Precise control of the As vapor pressure in the chosen growth system is required in order to maintain exact stoichiometry of the gallium arsenide compound during the growth process so as to achieve high mobility and crystal perfection.
One process utilized for commercial production of bulk gallium arsenide compound reduces arsenic related defects by providing a high arsenic vapor pressure. A source of arsenic is vaporized and reacts with gallium metal in a sealed quartz ampoule. This is followed by single crystal growth initiated and controlled by establishing thermal gradients for crystal growth across the ampoule (horizontal Bridgemen). The resulting semi-circular wafers are small and are not compatible with wafer handling equipment developed for the silicon IC industry. Furthermore, the wafers are of poor quality and still exhibit high inherent defect densities.
These substrates offer poor and unpredictable quality and exhibit high inherent defect densities. A common manifestation of poor quality is the formation of a conductive surface layer following a thermal annealing process step. Excessive surface conduction results in parasitic conductance paths between drain and source regions of adjacent transistors. These spurious conductance paths degrade device performance and cause incorrect circuit operation. This surface conductive layer may result from a redistribution of impurities during the thermal annealing process step.
The initial thrust of GaAs IC development activities required the growth of an epitaxial GaAs IC layer whose quality was directly dependent upon the quality of the substrate material. The large defect densities inherent in the Bridgeman crystal material would directly propagate into the epitaxial layer during its growth phase. The unpredictable and generally poor quality of this material proved a major stumbling block in GaAs IC development.
A much better method of growing crystals is to pull a crystal from a melt according to the Czochralski method which is the standard method for growing most of the device grade silicon. This method cannot be utilized to grow crystals from compounds having volatile components at the crystallization temperature such as gallium arsenide unless the melt is encapsulated with a thin layer of inert liquid such as boric oxide and pressurizing the chamber with an inert gas such as argon or nitrogen. This non-reactive high pressure atmosphere counterbalances the arsenic dissociation pressure. Typical gas pressures during operation can exceeed 30 atmospheres and require adequate safety precautions during use of the crystal pulling equipment.
The crystal pulling equipment is readily available for use in this liquid encapsulated crystal (LEC) technique. Larger crystals with lower levels of background impurities are produced than typical Bridgeman crystal material. Yields are also significantly higher and this method has proved quite successful and has become commercially feasible.
The LEC technique was first demonstrated by Metz et al in J. Appl. Phys., 33 (1962), 2016. The first reports of growth of GaAs and InAs crystals were by Mullin et al in J. Phys. Chem. Solids, 26 (1962), 782, and Bass et al in Symposium on GaAs (Institute of Physics and Physical Society of London), 1967, 41. The encapsulation layer must be less than 1.5 cm thick or control becomes very difficult as indicated by Thyagarajan et al in Indian J. of Pure and Appl. Phys., 17, Oct. 1979, 650. The temperature gradient at the interface is important since the growing crystal leaves the molten encapsulant environment where radiation heat transfer controls and enters the ambient inert gas where convective heat transfer processes control.
Russ in Solid State Technology, Aug. 1972, 29, 31 also reports on the need for temperature control and on the rapid heat loss associated with pulling a crystal from the melt through the thin liquid encapsulant into the gas atmosphere. Russ also reports that the growth process is inhibited if the B.sub.2 O.sub.3 layer is over 2 cm thick. Johnson produced a polycrystalline GaAs by liquid encapsulated floating zone (EFZ) by multiple pass melting of a GaAs rod in a floating zone of B.sub.2 O.sub.3 (J. Crystal Growth 30 [1975] 249-256).
The product of liquid encapsulated growth (LEC) is a single crystal containing substantial stress and stress artifacts (such as dislocations) due to extreme thermal gradients experienced by the growing crystal. The LEC produced crystals are also characterized by inclusions of gallium in the surface of the crystal due to evaporation of arsenic or phosphorous from the surface of the crystal while it is at elevated temperatures when it exits the boric oxide encapsulant. It is believed that crystal defects can be reduced if the thermal gradients in the growing crystal can be closely controlled to reduce the formation of defects and also to prevent the loss of one of the constituents of the compound until the crystal has cooled to a point where it is stable.